1. Field of the Invention
The present invention relates to hard disk drives and, more specifically but not exclusively, to hard disk drives that read data from hard disks that are segmented into alternating user sections and overhead sections.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
Conventional hard disks (HDs) for HD drives (HDDs) have a plurality of concentric tracks, each of which is segmented into alternating user sections for storing user data and overhead sections for storing overhead data that is used by HD drives to attain timing lock needed to read user data stored in the user sections. Traditional hard disks are formatted according to a segmentation scheme that interleaves user sections will full overhead sections that enable an HD drive to attain sufficient timing lock using any single, full overhead section such that the HD drive is then able to read user data stored in the user section immediately following that overhead section. Such traditional hard disks enable each user section to be read independent of every other user section on the hard disk such that user data can be stored in increments as small as a single user section.
FIG. 1 shows a representation of a single, exemplary (simplified) HD track 100 having 12 user sections 120(1)-120(12) interleaved with 12 intervening, full overhead sections 110(1)-110(12), where each full overhead section 110(i) includes:                A pad field 112 that functions as a Viterbi closure field;        A write splice (WS) field 114 that functions as a preamplifier closure field for write sessions;        A complete preamble timing recovery field (PTRF) 116 containing a known data pattern (e.g., a 2T pattern) that enables an HD drive to attain sufficient timing lock to read user data in the immediately following user section along the track; and        A sync mark (SM) field 118 containing a known data pattern that separates the end of the preamble and the beginning of the immediately following user section to enable the HD drive to locate bit 0 of the user data stored in that user section.The WS field covers the preamp closure (from the prior-written user data section and pad field) as well as the preamp opening (startup of the preamp for writing the following preamble and SM fields and user data section). Note that, in some cases, the WS field can also include so-called servo data that is completely unrelated to the read machine and is used by a separate machine for figuring out the track number and position along a track. In those cases, under normal operation, the WS field is skipped by both the read and write machines. Although exemplary track 100 of FIG. 1 is divided into 12 user sections 120 and 12 intervening overhead sections 110, tracks of typical hard disks are divided into many more user sections and intervening overhead sections.        
To read data from any user section 120(i) of track 100, an HD drive attains sufficient timing lock using the complete preamble 116 of the immediately preceding overhead section 110(i). Since each preamble 116 in each overhead section 110 of track 100 is a complete preamble, an HD drive can begin reading data from track 100 as soon as the next overhead section 110 is encountered. For the segmentation scheme of track 100 having 12 equal-sized user sections 120 interleaved with 12 equal-sized overhead sections 110, an HD drive, whose read head has a random position over track 100 when an HDD read session is initially requested, will have to wait, on average, about 1/24 of a rotation of track 100 to begin to read user data from the next encountered user section.
In general, it is desirable (i) to store as much user data as possible on a hard disk and (ii) to read stored user data as quickly as possible.